The invention relates to an exposure controller for a camera, and more particularly, to an exposure controller capable of assuring a proper exposure for taking a picture under a rear lighting condition which involves a large variation in the brightness level.
The photometry employed with cameras can generally be categorized, according to the distributed sensitivity to light incidence, into average photometry, a centrally biased photometry (or average photometry with emphasis on the central region) and spotwise photometry. These techniques of photometry are each advantageous in one aspect and disadvantageous in another. In general, depending on the condition of an object being photographed or the intent of photographic composition, a combination of spotwise photometry and average photometry may be used, or a centrally biased photometry which represents a median selection between the first mentioned photometry techniques may be used. Alternatively, an arrangement may be made to allow a choice between the centrally biased photometry and spotwise photometry or average photometry.
When a factor such as rear light produces a difference in the brightness level between a main subject to be taken such as a person located centrally on an image field and a marginal background which exceeds a given value, the main subject to be taken, for example, a person standing beside the window, will be subject to an underexposure if the average or centrally biased photometry is utilized.
For this reason, recent cameras are provided with a capability to effect both the spotwise and the average (centrally emphasized) photometry, permitting the spotwise photometry to be selectively used when taking a picture in the rear light. When using a camera having such switchable photometry arrangement, an unskilled photographer such as beginner may fail to take a picture successfully as a result of the underexposure caused by the utilization of the average photometry without noting that the object being photographed is in the rear light.
In a single lens reflex camera incorporating a focal plane shutter and capable of photometry of reflection from a film surface in which light from an object being photographed which passes through a taking lens and is reflected by the surface of the first blind of the shutter and a film surface is immediately determined to calculate an exposure period, which camera is commonly referred to as TTL (through-the-lens) direct photometry camera, an exposure control takes place by the photometry of light from an object being photographed which has passed through a diaphragm aperture which is actually controlled. Hence, the use of spotwise photometry with such camera in order to provide an exposure control is not desirable. Specifically, if the brightness of an object being photographed is low, the passage of such light through the diaphragm further reduces the level of light which is incident on a photometric circuit, causing a time delay in the leading end of an output from a photoelectric transducer element which is used for purpose of photometry, thus preventing an exact exposure period from being attained. Accordingly, the centrally biased photometry which permits a favorable photometry of almost any object being photographed is most frequently adopted with cameras which are designed for photometry of reflection from a film surface. Therefore, it would be desirable if it is possible to provide an exposure control based on the centrally biased photometry for those objects being photographed which are in the rear light, thus automatically providing a compensation for the rear light.